Sugarcane: Brown rust coming back – what you can do

Brown rust has not been a problem during the last two seasons for Louisiana sugarcane producers due to cold weather during the preceding winter. However, rust is the disease with the potential to cause the industry the greatest loss. Rust susceptible varieties are being grown on approximately two thirds of our acreage. A mild winter like the one we are currently experiencing will likely allow rust to survive and re-emerge as a problem. We could have a severe outbreak this spring. So, what can be done to minimize the impact of brown rust during 2012?

The best management practices for brown rust are: plant resistant varieties; diversify the varieties under cultivation; avoid excess fertility; remove green tissue from the row during winter; and apply fungicide. Which of these should be acted upon this spring? The varieties being grown are already established for the coming season. Clipping, or mechanically removing the green tissue in which the rust pathogen survives the winter, should have been done during January. This leaves fertility practices and fungicide application.

Growers hate the fact that brown rust is worst in the fields with the most vigorously growing cane. Research and field observation have indicated that fields with high nitrogen and phosphorus levels experience the most severe rust. Due to high fertilizer costs and research indicating lower nutrient requirements for the current varieties, growers have modified their fertilization practices. Therefore, excess fertility is usually not a factor.

Preventing rust from developing to damaging levels with fungicide application is the disease management practice that needs to be utilized during the coming spring. What are some guidelines that will make this practice most cost effective? Where and when should you apply a fungicide?

The following conditions identify the fields that are likely to be hit hard by rust (the fields that need to be monitored closely).

• Rust infection is evident on older leaves and beginning on younger leaves of plants with most advanced growth. These plants may be along a tree line or ditch bank.

• Rust infection becomes evident from late March until early June. The earlier the disease begins and longer it lasts, the more yield loss it will cause, and the more economic benefit will result from fungicide application.

HoCP 96-540 and L 99-226, the two most widely planted varieties, are both now susceptible to brown rust. The worst rust was seen during the fall in advanced fields of L 99-226. LCP 85-384 and Ho 95-988 are both highly susceptible to rust, but there is very little plant cane of these varieties. L 97-128 has exhibited moderate susceptibility to brown rust, but there is little plant cane of this variety. We do not have as much information about L 01-283, but it may now be susceptible. Plant cane of L 01-283 should be monitored. Vigorous stubble fields of susceptible varieties may develop rust early enough this spring to benefit from a fungicide application, but rust will develop later in stubble fields. Use the same guidelines to identify stubble fields that need to be monitored and possibly treated.

Brown rust begins first and is most severe in plant cane growing in light textured soil. The very first rust symptoms are usually seen in portions of fields that were protected from freezes along tree lines. In other fields, rust symptoms will begin to show on the younger leaves of plants with the most advanced growth, at the ends of rows or along ditchbanks. Growers should watch these fields and advanced plants within those fields to use them as an “early warning system”.

The big question this spring is: How do you determine when to apply fungicide? Rust will be evident on older leaves of plants very early. The rust needs to build up on the older leaves before it moves onto the young leaves. Rust lesions can typically be found on older leaves for several weeks before the epidemic that will reduce yield begins on the younger leaves. This means seeing lesions on the older leaves does not indicate an immediate need to apply fungicide. We have not been able to use a calendar basis for timing fungicide application because of the high variability in conditions between seasons in Louisiana.

Prior to this year, there has been no need for fungicide application before mid-April even in the southernmost areas of the industry. This is a very unusual year, and the need for a fungicide application may occur at the earliest date yet. Again, the appearance of obvious rust on younger leaves (rust symptoms that can be seen from outside the field) of the more advanced plants in the most vigorous fields will indicate that a damaging rust epidemic is about to begin in that field. Once this occurs, rust may begin to develop on the younger leaves of all plants in the field within a few days.

It is likely that the epidemic will occur over an extended period in some plant cane fields. Only two fungicide applications are allowed by the label, and two applications may be needed. How should two applications be timed? We want to have protection during the time when the crop is beginning to respond to warming conditions and the availability of fertilizer. With a three-week protection period from a single spray, the time for most benefit from protection will probably be the second half of April and the month of May.

What happens if a field has already turned red from rust before a fungicide is applied? Fungicide protects leaves from infection. It does not cure infection that has already taken place. Application of fungicide to fields that have already turned red from rust infection on the young leaves will provide protection to the subsequently developing leaves. The crop will “green-up” in the weeks after fungicide application as new leaves emerge and expand. Research has shown that the potential yield benefit from a single application in this “rescue” situation will be less. Economically, this type of application will be closer to a “break-even” proposition. However, if this application takes place early enough that a second application will be needed for the rest of the spring, then a first application should definitely be made as soon as possible to begin the extended two application protection window.

The fungicide available to control brown rust is Headline. The label allows two applications at a rate of 9 ounces of formulated product per acre. It should be applied in enough water to provide good leaf coverage – 15 gallons of water per acre or more. Addition of a surfactant will improve coverage. Application of Headline on a 36 inch band will provide good coverage of plants during the spring that have not grown enough to cover the row middle. A banded application will reduce the cost of treatment. Second applications to more advanced cane will probably need full broadcast treatment.

A brown rust epidemic is likely during the spring of 2012. A large portion of the industry is planted to rust susceptible varieties. Yield benefits as great as 10 tons have resulted from protecting fields from extended brown rust infection.

Well-targeted and timed application of Headline will definitely provide a positive economic return. Fungicide should be applied as needed on a field/block basis to maximize the economic benefit of treatment.

Your crop consultant can help keep you informed about the developing rust situation on your farm this spring so that you can decide which, if any, fields need Headline treatment. All Louisiana sugarcane producers need an understanding of where and when to apply fungicide to prevent significant yield loss.

More information is available on the LSU AgCenter website, www.lsuagcenter.com. Search for: Best management practices to minimize the impact of brown rust on sugarcane. Additional information can be obtained from the Louisiana Cooperative Extension Service.